Regenerative amplifiers utilizing chirped pulse amplification (CPA) have been the dominant means for obtaining higher pulse energies with picosecond and femtosecond pulse duration. The CPA regenerative amplifier was first demonstrated for picosecond amplification in Nd:glass in by Gerard Mourou and Donna Strickland, Compression of Amplified Chirped Optical Pulses, Optics Communications 56 (3): 219-221, Dec. 1, 1995.
The development of broad band solid state lasers as regenerative amplifiers was first demonstrated by D. Harter and P. Bado, Wavelength Tunable Alexandrite Regenerative Amplifier, Applied Optics 27 (21): 4392-4395, Nov. 1, 1988 and by D. Harter, O. Montoya, J. Squier and W. Rapoport, Short Pulse Generation in Ti: doped materials, CLEO 1988 PD6. These systems were utilized for femtosecond amplification and were reported by M. Pessot, J. Squier, G. Mourou and D. Harter, Chirped-Pulse Amplification of 100-Fsec Pulses, Optics Letters 14 (15): 797-799, Aug. 1, 1989 and by J. Squier, F. Salin, G. Mourou and D. Harter, 100-FS Pulse Generation and Amplification in Ti—Al2O3, Optics Letters 16 (5): 324-326, Mar. 1, 1991.
It is the Ti:sapphire regenerative amplifier that has been the dominant method for obtaining femtosecond pulses in the microjoule to millijoule range. These systems have been made more practical by using modelocked fiber lasers as the source of the short pulses, as first reported by A. Hariharan, M. E. Fermann, M. L. Stock, D. Harter and J. Squier, Alexandrite-pumped Alexandrite Regenerative Amplifier for Femtosecond Pulse Amplification, Optics Letters 21 (2): 128-130, Jan. 15, 1996 and later patented by Clark in U.S. Pat. No. 5,530,582 “Fiber source for seeding an Ultrashort optical pulse amplifier.” This seeding has been studied in a number of Ti:sapphire regenerative amplifiers, as reported by A. Hariharan, D. Harter, T. S. Sosnowski, S. Kane, D. T. Du, T. B. Norris and J. Squier, Injection of Ultrafast Regenerative Amplifiers with Low Energy Femtosecond Pulses from an Er-Doped Fiber Laser, Optics Communications 132 (5-6): 469-473, Dec. 15, 1996.
Alternative sources for microjoule level femtosecond pulses are emerging by all fiber designs as first described by M. E. Fermann, A. Galvanauskas and D. Harter, All-Fiber Source Of 100 nJ Subpicosecond Pulses, Appl. Phys. Letters Vol. 64, 11, 1994, pp. 1315-1317. During the past decade, there has been intensive work in making such systems practical. The recent results were reported by M. Stock, H. Endert and R. Patel, Time-Tailored Laser Pulses: a New Approach for Laser Micromachining and Microfabrication Processing, SPIE Photonics West 2003, San Jose and SPIE Publication # 4984-28. Such systems should be useful for lower energy applications such as LASIK as reported by T. Juhasz, H. Frieder, R. M. Kurtz, C. Horvath, J. F. Bille and G. Mourou, Corneal Refractive Surgery with Femtosecond Lasers, IEEE Journal Of Selected Topics In Quantum Electronics 5 (4): 902-910, July-August 1999.
However, for higher pulse energies, the regenerative amplifiers will continue to dominate because of practical uses such as micromachining as described by Xinbing Liu and Gerard Mourou, Ultrashort Laser Pulses Tackle Precision Machining, Laser Focus World, August 1997, Vol. 33, Issue 8, page 101.
For the micromachining applications, more industrially compatible regenerative amplifiers are being developed. These systems are based on Nd: or Yb: doped materials, rather than the Ti:sapphire that has dominated the scientific market. There are two basic reasons for this change. Commercial markets typically do not require the shorter pulses that can only be obtained from the Ti:sapphire regenerative amplifier and the Nd: and Yb: based materials can be directly diode pumped, which makes these systems more robust and less expensive. An unresolved technical issue for Nd: or Yb: based regenerative amplifiers is the need for an equally robust seed source for the femtosecond or picosecond pulses. The present lasers are mode-locked solid-state lasers with questionable reliability. It would be preferable to have a robust fiber laser similar to that which has been developed for the Ti:sapphire regenerative amplifier.
Historically, both Nd: doped crystals and glasses have been the laser material for most solid-state lasers. The only commercially available regenerative amplifiers for picosecond or femtosecond pulse besides Ti:sapphire have been Nd: based materials. Ti:sapphire can produce much shorter pulses. The Nd: crystalline materials such as Nd:YAG, Nd:YLF and Nd:Vanadate produce pulses around 10 picoseconds. The Nd:glass regenerative amplifier produces pulses around 0.5-2.0 picoseconds. The Nd:glass regenerative amplifier is the source used in the Laser-Assisted In-Situ Keratomileusis (LASIK) application and the laser is described by C. Horvath, A. Braun, H. Liu, T. Juhasz and G. Mourou, Compact Directly Diode-Pumped Femtosecond Nd: glass Chirped-Pulse-Amplification Laser System, Optics Letters 22 (23): 1790-1792, Dec. 1, 1997.
Recently, there has been interest in replacing Nd: lasers with Yb: lasers. The reasons are related to diode pumping. The absorption transition is broader than the absorption transition in Nd: lasers, so it is easier to tune the laser diodes to the transition. It is also possible to dope the crystals heavily so the diodes are absorbed in a small area leading to higher gain. The final reason is that the transition has a small quantum defect from the laser transition so more efficient lasing is obtained with less heat deposition. The broader transitions also allow shorter pulse generation. A very good review of the advantages of Yb: materials for short pulse is reported by J. Nees, S. Biswal, F. Druon, J. Faure, M. Nantel, G. A. Mourou, A. Nishimura, H. Takuma, J. Itatani, J. C. Chanteloup and C. Honninger, Ensuring Compactness, Reliability, and Scalability for the Next Generation of High-Field Lasers, IEEE Journal Of Selected Topics In Quantum Electronics 4 (2): 376-384 March-April 1998.
However, Yb: materials have some idiosyncrasies that make them more challenging to use. Typically, the regenerative amplifier active material is a four level transition and the gain shape is very constant. This is true with Nd: doped materials and Ti:sapphire. An early exception is alexandrite, which has a quasi-three level lasing transition. The spectrum is quite stable but can be tuned for example by the excitation level or by large temperature differences. This was illustrated where the temperature of alexandrite can be used so the gain spectrum of one crystal overlaps the absorption spectrum of another crystal. A room-temperature alexandrite laser can pump another hot alexandrite laser as was disclosed by A. Hariharan et al., Alexandrite-pumped Alexandrite Regenerative Amplifier for Femtosecond Pulse Amplification, Optics Letters 21 (2): 128-130, Jan. 15, 1996.
Wavelength shifting of the gain spectrum is also seen in Yb: doped regenerative amplifiers as is shown in FIG. 6 from H. Liu, J. Nees, G. Mourou, S. Biswal, G. J. Spuhler, U. Keller and N. V. Kuleshov, Yb: KGd(WO4)2 Chirped-Pulse Regenerative Amplifiers, Optics Communications 203 (3-6): 315-321, Mar. 15, 2002. In this case, the seed femtosecond pulse was at 1027 nanometers and the output was 1038 nanometers. Thus, to obtain the optimized wavelength, one needs to inject a different wavelength in order to obtain the optimized wavelength. However, Table 1 shows that the optimum wavelength is dependent on loss (OC) and can vary as much as 28 nanometers. In fact, the optimum wavelength changes by 1-3 nanometers per percentage of loss!
For Yb:glass, the spectral shift of the input seed to the output is shown clearly in FIG. 4 from H. Liu, S. Biswal, J. Paye, J. Nees, G. Mourou, C. Honninger and U. Keller, Directly Diode-Pumped Milliyoule Subpicosecond Yb:glass Regenerative Amplifier, Optics Letters 24 (13): 917-919, Jul. 1, 1999. This behavior is partially explained from the family of curves in FIG. 2 from C. Honninger, R. Paschotta, M. Graf, F. Morier-Genoud, G. Zhang, M. Moser, S. Biswal, J. Nees, A. Braun, G. A. Mourou, I. Johannsen, A. Giesen, W. Seeber and U. Keller, Ultrafast Ytterbium-Doped Bulk Lasers and Laser Amplifiers, Applied Physics B-Lasers and Optics 69 (1): 3-17, July 1999. The gain peak changes with excitation dramatically and as the seed pulse is amplified the excitation value is changing with the saturation of the laser material. Table 2 shows that, with a 7% change in loss, the spectral peak moves 30 nanometers and the spectral width changes 65% in Yb:glass. Such specification changes need to be considered for long-term operation where the loss does increase with time.
It should be clear that it is difficult to design the seed laser for Yb: regenerative amplifiers due to the large changes in the center line width and bandwidth caused by pumping level, losses and the Yb: host material.
There are additional differences with Yb: materials that make them challenging. One is the round trip gain is lower compared to that in a Ti:sapphire regenerative amplifier. In order to obtain the required gain, there must be additional round trips in the regenerative amplifier and with more round trips the system is more susceptible to changes in loss due to environment. Also, a 1% change in the round trip loss is a small change in the overall gain when the round trip gain is 200% in Ti:sapphire, but is a very large change in the overall gain when the round trip gain is 10% as is the case in these Yb:doped regenerative amplifiers.
The thin disk configuration is being considered for high power industrial applications. The thin disk refers to doped lasing material where the pump laser is passed multiple times for absorption. The thin disk is mounted on a conductive mirror so that the heat is removed perpendicular to the face of the crystal rather than radially removed in the typical rod configuration. In a thin disk regenerative amplifier, short pulse amplification has been shown without the use of chirped pulse amplifiers. This is possible due to the shorter propagation distances in and larger spot sizes in the thin disk compared to conventional rod solid state desings. This work has been reported by C. Honninger, I. Johannsen, M. Moser, G. Zhang, A. Giesen and U. Keller, Diode-Pumped Thin-Disk Yb: YAG Regenerative Amplifier, Applied Physics B-Lasers And Optics 65 (3): 423-426, September 1997. More recent results have been published by M. H. Niemz, A. Kasenbacher, M. Strassl, A. Backer, A. Beyertt, D. Nickel and A. Giesen, Tooth Ablation Using a CPA-Free Thin Disk Femtosecond Laser System, Applied Physics B-Lasers And Optics 79 (3): 269-271, August 2004. The laser system is described in International Application No. WO 04/068657 A1. In these experiments, a Yb:YAG and a Yb:glass mode-locked laser have been used as the seed source, respectively. These sources have not met the required stability for an industrial source and there has been great interest in developing stable mode-locked fiber lasers for this application.
A fiber laser has been built to seed an Yb: based regenerative amplifier. A brief description of this fiber laser for this application is given in U.S. Pat. No. 6,760,356, the disclosure of which is incorporated by reference in its entirety. This fiber laser is based on the identical erbium mode-locked laser that operates at 1.55 micrometers and is converted to 780 nanometers for seeding Ti:sapphire regenerative amplifiers. However, the frequency is converted to 1040 nanometers for injection seeding the Yb: regenerative amplifier. This fiber laser system is described in detail in co-pending U.S. application Ser. No. 09/576,772 entitled MODULAR, HIGH ENERGY, WIDELY-TUNABLE ULTRAFAST FIBER SOURCE and was reported by M. E. Fermann, A. Galvanauskas, M. L. Stock, K. K. Wong and D. Harter, Ultrawide Tunable Er Soliton Fiber Laser Amplified in Yb-Doped Fiber, Optics Letters, Vol. 24, No. 20 pp. 1428-1430, Oct. 15, 1999.
However, it is now desirable to have a simpler laser and thus more cost effective solution. An early experiment in injection seeding with a fiber laser is described by M. Hofer, M. H. Ober, F. Haberl, M. E. Fermann, E. R. Taylor and K. P. Jedrzejewski, Regenerative Nd Glass Amplifier Seeded with a Nd Fiber Laser, Optics Letters 17 (11): 807-809, Jun. 1, 1992. However, this fiber laser required 6 prisms intracavity and over a meter of free intracavity spacing, and thus, this laser is no more than a proof of principle demonstration.
Co-pending U.S. application Ser. No. 09/576,772, filed May 23, 2000, which is assigned to the common assignee and the disclosure of which is incorporated by reference in its entirety, discloses the use of a Yb: oscillator, which can be used for the seed source of regenerative amplifiers with the additional system considerations described here.